Substrate specificity in the Devonian tabulate coral Pleurodictyum

The tabulate coral Pleurodictyum americanum Roemer has been cited as an example of a host-specific organism occurring exclusively on the shells of gastropods, particularly Palaeozygopieura hamiltoniae (Hall). Examination of over 1600 specimens of P. americanum, from the Middle Devonian Hamilton Group of western New York, reveals additional complexities which require reinterpretation. While substrate selectivity for Palaeozygopieura shells is evident in all 42 subsamples, a variety of other substrates were also utilized by Pleurodictyum including corals, brachiopods, other molluscs and pebbles. Recent scleractinian corals inhabiting soft bottoms show similar substrate preference, selecting for the tubes of live serpulids, or gastropod shells (invariably with a secondary sipunculid host), but also occasionally settling on unoccupied shells or pebbles. Shell surfaces of P. hamiltoniae, preserved as external molds on the Pleurodictyum epitheca, exhibit encrustation by worm tubes and bryozoans as well as borings and mechanical shell damage, suggesting that these were not the shells of live gastropods. However, the invariant aperture-downward orientation and the high degree of selectivity of P. americanum strongly suggest that the shells were occupied by secondary hosts. □ Substrate specificity, commen-salism, tabulate coral, gastropod, sipunculid, Devonian, Hamilton Group, New York.     

Symbiosis between gastropods and bryozoans in the late Ordovician of Cumbria, England

Five examples of symbiosis between gastropods and trepostome bryozoans are described from the Ashgill (late Ordovician) of Cumbria (England). The gastropods are invariably found associated with the bryozoans. whereas the bryozoans may be free-living. Encrustation is considered to have taken place, in most cases, on mature, living gastropod shells, resulting in the trepostome utilizing the shell as a surrogate basal disc. Three specimens show encrustation on an empty gastropod shell, causing the trepostome to develop a basal disc similar to non-encrusting forms. The bryozoan colony probably benefited from the symbiotic relationship by increased water flow over the colony, caused by gastropod locomotion, whereas the gastropod was afforded protection from predators. The new species Spiroecus nidhoeggi. Diplotrypa hvergelmi and Monotrypa fontinalis are described.     

Symbiosis, competition, and physical disturbance in the growth histories of Pliocene cheilostome bryoliths

Free-living (unattached) subspherical bryozoan masses (bryoliths) in Pliocene tidal channel deposits of the Imperial Formation of southeastern California show complex intra- and interspecific interactions during their accretionary growth. Ranging up to 10 cm in length, the bryoliths are composed almost exclusively of the anascan cheilostome Biflustra commensale. Approximately 50% of the bryoliths are nucleated on cerithiid or muricoid gastropod shells; secondary occupants (presumably pagurid crabs) determined the subspherical growth and associated epibionts of all of these specimens. Evidence for crab occupation includes the thick and relatively symmetrical bryozoan overgrowths that form short tubes extending from the aperture, thinning and pinchout of laminae on the undersides of bryoliths (wear facets), and the distinctive borings (Helicotaphrichnus) of symbiotic worms. In some instances, shells were infested by bryozoans and other encrusters before death of the gastropod, but these overgrowths are thin relative to hermit-associated bryozoan colonies. Episodic hermit abandonment, indicated by extensive erosion of the bryolith and/or its colonization by a more diverse epibiont assemblage including oysters and serpulids, was more frequent among bryoliths nucleated on the largest and most fouled gastropod shells; it was also more frequent among bryoliths in the relatively high-energy tidal channel thalweg than among those associated with oyster thickets on muddy channel margins. Bryoliths nucleated on other shell substrata are similarly thick, but have more irregular stratigraphies including more sedimentary inclusions, more borings, and fewer encrusting epibionts. Pebbles of crystalline basement rock are also encrusted by B. commensale, but only thinly. All of these bryoliths not inhabited by crabs are limited to the channel thalweg. As many as four distinct colonies of B. commensale could coexist on a single bryolith; lines of competitive standoff between colonies are marked by mineralized walls and topographic ridges on the bryolith exterior, and by teepee-like structures in cross-section. These standoffs were preferred sites of infestation by other epibionts and were remarkably stable in position on bryoliths with continuous hermit occupation. Bryoliths that suffered repeated abandonment by hermits, or that depended entirely upon chance reorientation, are characterized by highly unstable standoff positions, reflecting scramble competition under less predictable conditions. These circumstances were most common among large bryoliths and among those in channel thalwegs.     

MORPHOLOGY AND BEHAVIOR OF CRABS AND GASTROPODS FROM LAKE TANGANYIKA,AFRICA: IMPLICATIONS FOR LACUSTRINE PREDATOR-PREY COEVOLUTION

The shells of most lacustrine gastropods are typically small, weakly calcified, and modestly ornamented to unornamented. Similarly, most lacustrine crabs are usually small detritivores with weak chelae. A number of invertebrate taxa in Lake Tanganyika, however, deviate from these generalities. This study explores a predator-prey coevolution model as an explanation for the large, heavily calcified, and ornate gastropods and the robust, durophagous crabs of Lake Tanganyika. The endemic thiarid and viviparid gastropods from Lake Tanganyika have significantly thicker shells and higher frequencies of terminal apertural lip thickening than closely related cosmopolitan taxa from outside the lake. Tanganyikan gastropods also display considerably higher incidence of shell repair, following nonlethal shell damage, than cosmopolitan taxa of the same families. There is a strong positive correlation between gastropod apertural lip thickness and shell repair frequency among all the gastropod species analyzed. The endemic Tanganyikan potamonautid crab Platytelphusa armata (a molluscivore) possesses larger, more robust crushing chelae than other African potamonautid or potamonid crabs. In contrast with the cosmopolitan African crabs, the Tanganyikan crabs display molariform, rather than serrate dentition on their crushing chelipeds. In shell-crushing experiments, the Tanganyikan gastropod shells were an order of magnitude stronger than typical lacustrine gastropod shells, many well within the range of tropical marine gastropod shell strengths. Predation experiments with the endemic gastropods Spekia, Neothauma, Lavigeria spp., Paramelania spp. and the crab Platytelphusa armata showed that increased size, apertural lip thickness or shell sculpture reduced the successful predation rate of P. armata. Crabs with large chelae have a greater ratio of successful: unsuccessful attacks than crabs with small chelae. Among cases of successful predation, crabs with large chelae employed predation methods that required less time and energy (such as crushing the shell in the cheliped) than the methods employed by crabs with small chelae (such as peeling the shell from the aperture or the spire). The morphological, shell-crushing, and aquarium experiment data, considered in concert, provide strong support for the idea that the endemic gastropods and crabs of Lake Tanganyika have coevolved over the past 7 million years.     

Autecology of Labrocuspis, a Middle Devonian omphalotrochid gastropod

Labrocuspis kobayashii , a Middle Devonian euomphalacean omphalotrochid gastropod from Japan has a wide callus pad on its base similar to that seen in the living trochid Umbonium. L. kobayashii could support the shell over the cephalopedal mass in a similar fashion to that seen in the Umbonium species. This reconstruction indicates that the labral projection in the outer lip was above the head, and the opisthocyrt basal lip and the sinus in the outer lip were loci of inhalant and exhalant currents, respectively. Therefore, the animal is assumed to have had only one ctenidium (the left). The presence of a wide callus pad is indicative of an infaunal life habit for the Umbonium species, Labrocuspis and some Paleozoic gastropods; clamping of the shell against the foot is also suggested for some omphalotrochid gastropods. These features indicate an additional adaptive strategy employed by euomphalaceans.     

Host‐specific acrothoracid barnacles on Middle Devonian platyceratid gastropods

Borings, attributed to acrothoracic barnacles, occur on the platyceratid gastropod Naticonema lineatum (Conrad) from the Middle Devonian Hamilton Group of western New York and rarely in specimens as old as the Early Devonian. These latter are the oldest known acrothoracid borings are in the fossil record. The borings are consistently developed as laterally compressed, inequilateral pouches exclusively on these gastropods, commonly as dense infestations.

Naticonema shells yielding borings typically occur associated with partially articulated qrinoid remains, and they are sometimes found attached to crinoids in a manner similar to coprophagous Platyceras. In addition to barnacle borings, Naticonema shells often bear thin encrustations of bryozoans which are usually perforated by these borings but sometimes also overgrow them. Barnacles bored live hosts; gastropods prevented shell penetration by producing cyst‐like secondary secretions of calcite beneath acrothoracid boreholes.

The relative antiquity of these borings and their association with coprophagous platyceratids makes their discovery particularly significant in revealing aspects of the early ecology of barnacles. Attachment to the host commensal gastropods was one of the first successful life modes of these crustaceans prior to their later diversification to other habitats. Mississippian and Pennsylvanian occurrences of similarly bored gastropods demonstrate continuity of the barnacle‐gastropod‐crinoid ecological association from the Middle to Late Paleozoic.     

Unusual skeletal morphology and systematic description of a new Devonian cryptostome bryozoan from the Western Sahara

Palaeozoic bryozoan colonies display a large variety of skeletal elements. Various types of rod-like styles are mainly built by laminated and non-laminated (hyaline) skeleton. Typical styles consist of hyaline cores and surrounding laminated sheaths. They usually protrude on the colony surface as spines. In a new bryozoan genus from the Middle Devonian of the Western Sahara described here, styles do not protrude on the colony surface and are embedded within the laminated skeleton. They consist of fibrous material. This new type of styles, unknown from other bryozoans, is here called a “cryptostyle”. A special characteristic of these structures is their intensive reddish-brown colouration. This colouration is apparently caused by the presence of ferric iron between the individual fibres. The function of cryptostyles was apparently weight reduction and stabilising the skeleton. The general morphology of the new bryozoan, Cryptostyloecia hexapuncta gen. et n. sp., implies its systematic position within ptilodictyine cryptostomes, specifically the Family Ptilodictyidae Zittel, 1880.     

The encrustation of fossil and recent sea-urchin tests: ecological and taphonomic significance

A comparison of fossil ( Echitwlampas sp. from the Lower Miocene Zogelsdorf Formation, Austria) and Recent ( Schizaster canaliferus from the northern Adriatic Sea) irregular sea-urchin tests shows that, upon their death, burrowing echinoids can serve as a substrate for a dense epigrowth. Size, shape, stable orientation, and surface residence-time were identified as key factors governing encrustation. The encrusters on fossil Echinolampas were bryozoans, serpulid polychaetes, barnacles, and coralline algae, while the recent material was initially encrusted by serpulids and hydrozoan colonies, and ultimately covered by the full range of sessile, hemi-sessile, and vagile species characterizing the benthic community in the Adriatic. In Echinolampas , epigrowth was more abundant on the lower (oral) surface; this specific distribution was echoed in S. canaliferus , where epigrowth started on the bottom side and grew upward. This indicates that the tests have a stable orientation and a surface residence-time long enough to allow intense encrustation. A taphonomic model is developed, and the role of encrustation on such special substrates for overall community structure is discussed. The Recent/fossil comparison provides new insights for both fields of study: the recent material indicates the role of soft-bodied faunas as well as the complexity of small-scaled ecological processes; the fossil material reflects many of the above phenomena and adds important taphonomic details on the fate of encrusted biogenic structures and on encruster growth patterns and distributions.     

Mode of life of the Silurian uncoiled gastropod Semitubina sakoi n. sp. from Japan

Semitubina sakoi n. sp. from the late Silurian of Japan represents the second species of this genus and also the first record of a Silurian gastropod in Japan. The gastropod shells occur in a thin mudstone bed and were found to be encrusted exclusively by corallites of ? Favosites sp. These corallites reveal that encrustation proceeded as the gastropod shells grew. The ecological relationship between the two organisms is considered to be symbiotic. This mode of life allowed the coral to live on a muddy substrate because clear sea water passed over the colony as the gastropod moved along. The gastropod benefited from this relationship by being protected from shell-boring or shell-crushing predators by the encrusting corallite. In Semitubina sakoi the body whorl is separated from the penultimate one by a considerable gap in a later growth stage and S. sakoi has been cited as one of the uncoiled gastropods. The uncoiling of this gastropod results primarily from rapidly increasing whorl translation rate in the latest growth stage. Taking the symbiotic relationship with ? Favosites sp. into consideration, a deposit feeding or benthic scavenging mode of life is suggested for this gastropod.     

An unusual trochiform gastropod from the upper Ordovician of Sweden

An unusual trochiform gastropod, Semizona bella gen. et sp. nov., is described from the Boda Limestone carbonate mounds (upper Ordovician, Ashgill) of central Sweden. A second species, S. glindmeyeri (Rohr, 1996), is recognised from the Ordovician (Whiterockian) of Nevada. The shell shape and the strongly prosocline tangential aperture of Semizona suggest that balancing of the shell on the head-foot mass was accomplished by tilting of the axis of coiling of the shell to about 65 degrees with 10–30 degrees of regulatory detorsion. The rounded aperture allowed straight contraction of the retractor muscles, suggesting clamping behaviour often associated with a sedate, grazing snail. This agrees with the environmental setting, which suggests a hard substrate with rich microbial growth. Besides clamping, the subsutural nodes and thick shell were probably effective against predation; repaired injuries indicate failed predatory attacks. Semizona shows morphological similarities with some pleurotomariin vetigastropods, and with the family Pseudophoridae Miller, 1889.     

Finite element analysis of a double membrane tube (DMS-tube) and its implication for gastropod shell morphology

Molluscan shells, including those of Gastropoda, are formed by accretionary growth at the mantle edge. The mantle is a thin membrane of skirt-like shape, which extends minutely beyond the aperture, and its edge adds a shell increment to the aperture margin so that each increment copies a configuration of the mantle edge at that time. Thus, regulation of shell morphogeny is almost equivalent to the factors which control the mantle form at the moment of shell growth. Form of the mantle skirt is considered to be kept in a state of balance between the force of its internal stress and forces acting on it such as fluid pressure or muscle contraction. The expansion behavior of the mantle skirt has been numerically analyzed by using an elastic model (DMS-tube), which represents the fundamental structure of the mantle tissue as a double membrane structure with internal springs (DMS). Four characteristic expansion patterns of the DMS-tube have been detected: (1) general outward expansion; (2) developing a ridge-like fold on an initial longitudinal protrusion of the tube edge; (3) drastic shift of the expanded state from a uniformly curved to an elliptical shape in outline, owing to the existence of a fixed boundary condition on the tube wall; and (4) constricted protrusion on the open region of the shell wall surrounding the DMS-tube. These results have the potential for answering the following questions relating to the morphogenesis of gastropod shells. How does the mantle skirt usually make contact with the inner surface of the shell wall so as to ensure continuous accretion of shell materials to the aperture margin? What is the cause of spiral ridges? Why do open coiling or minimally overlapping shells have generally circular apertures, while shells with apertures overlapped by whorls have non-uniformly curved apertural lips? What is the cause of long closed spines and why do they always appear on spiral ridges?     

Substrate specific associations of epebionts on Middle Devonian brachiopods: Implications for paleoecology

A detailed study of over 2500 host brachiopods, from the Middle Devonian Hamilton Group of New York State, revealed distinct patterns of epibiont encrustation, that provide insight into taphonomy and paleoautecology of the host brachiopod shells and depositional environments. The concavo‐convex orthid, Tropidoleptus carinatus (Conrad), as well as strophomenid, and smooth athyrid brachiopods are among the most heavily encrusted. However, terebratulids of nearly identical size and shape are relatively clean of epibionts. This selective distribution strongly suggests that epibionts were discouraged from settling on punctate brachiopods. Brachiopods with small spines and frills were also nearly clean of epibionts, possibly because of entrapment of a mud layer, which made the outer layer of the host inhospitable for larval settling. Concavo‐convex taxa reveal high percent coverage and diversity of epibionts on the convex valve, which probably rested on the substrate during the life of brachiopod. This pattern is observed even on brachiopods that were buried with the convex valve downward. This implies complex post‐mortem histories involving multiple episodes of reorientation and colonization.     

Divergent shell shape as an antipredator adaptation in tropical land snails

Although many land snails exhibit amazingly divergent shell shapes in the tropics, the functions of these remain obscure. Here we show that a modified aperture shape acts as an impediment specifically to predation by a snail-eating snake. Pareas iwasakii (Colubridae: Pareatinae) uses a unique method to feed on land snails: the snake extracts the soft body from the shell through the aperture by alternately retracting its mandibles. The snail Satsuma caliginosa (Camaenidae: Camaeninae) has apertural variation in regard to the presence of snail-eating snakes. Our experiments demonstrated that the distorted aperture mechanically impeded predation by this gape-limited predator, interrupting the mandibular movements. In contrast, congeneric snails with round apertures did not escape predation by snakes. The paleobiogeography of the focal area indicates that the subspecies Satsuma caliginosa picta, which does not have apertural modification, was derived from a defensive ancestor after the extinction of snail-eating snakes. Our study suggests a possibility that snail-eating snakes are responsible for divergent shell shapes in a variety of tropical land snails.     

滇东泥盆系几个含鱼层的时代和对比

从鱼化石的角度对滇东泥盆系下述含鱼地层进行了分析,并认为:1)曲靖地区下泥盆统徐家冲组与坡松冲组或四川的平驿铺组、广西的那高岭组层位相当,时代为布拉格期;其层位低于含 duyunolepids-Kueichowlepis鱼类组合的贵州乌当组或舒家坪组,后者时代为埃姆斯期。2)滇东中泥盆统含两个不同时期的含鱼层,其中含Yinosteus-Wudinolepis鱼类组合的武定地区旧城组属中泥盆统早期,该鱼类组合现仅知分布于武定地区。而中泥盆统晚期含鱼地层,在曲靖地区由下而上包括穿洞组、上双河组和海口组三个岩石组;其中穿洞组＋上双河组与武定的鱼子甸组对比,而海口组与曲靖组为同时异相;武定茶花箐组与曲靖组对比。3)文中时上述地层中的某些鱼化石的层位或鉴定作了评述或厘定。     

Occluded Umbilicus In The Pinacitinae (Devonian) And Its Palaeoecological Implications

Eifelian (Middle Devonian) ammonoids of the Pinacitinae Hyatt, 1900 ( Exopinacites , Pinacites )with preserved shell structures from the eastern Anti–Atlas (Morocco) have revealed unusual morphological features. The Pinacitinae belong to the earliest ammonoids which closed their umbilici. As an approach to an interpretation of these structures, the representatives of the subfamily Pinacitinae ( Exopinacites singularis , Pinacites jugleri , P. eminens ) are compared with other ammonoids, e.g. Acrimeroceras , Araucanites , Clistoceras , Gaudryceras , Nathorstites , Prolobites , and Synpharciceras , which produced umbilical plugs and covers. Some of these are comparable in structure to Nautilus pompilius and N. belauensis . In contrast to all of these taxa, the lateral shell wall of the Pinacitinae reached the centre of the umbilicus and formed an umbilical lid. The umbilical shell wall rests on the umbilical lid of the previous whorl. This construction probably had the advantage that it improved the hydrodynamic properties of the conch, along with the oxyconic conch shape and the approximately horizontal orientation of the aperture.     

The functional significance of aperture form in gastropods

Aperture form of marine prosobranch gastropods has evolved under the influence of a number of different selective forces, including: generation of shell form; protection from predation; accommodation of the foot during clamping behavior: and accommodation of water currents in and out of the mantle cavity. Aperture form correlates positively with foot shape in most gastropods and foot shape, in turn, correlates moderately well with substrate preference. Almost all gastropods that have non-round apertures elongate the aperture parallel to the foot so that water currenth tend to flow anteriorly to posteriorly. Fresh-water pulmonates have responded to somewhat different stresses. They exhibit clamping behavior and thus show correspondence between foot shape and aperture shape. They show less apertural strengthening as crab (or crayfish) predation is less of a factor and presumably because calcium carbonate is less available. They also lack anterior-posterior apertural elongation due to the absence of water currents through their mantle cavity. Due to the absence of mantle cavity water currents and clamping behavior, terrestrial gastropods do not show the apertural modifications associated with these two factors. In addition. few adaptations of apertural form are present to resist predation. Instead, many of the apertural modifications of terrestrial pulmonates seem to be concer-ned with the problems of water loss during estivation.     

Life mode of in situ Conularia in a Middle Devonian epibole

Abstract: Exceptionally abundant specimens of Conularia aff. desiderata Hall occur in multiple marine obrution deposits, in a single sixth‐order parasequence composed of argillaceous and silty very fine sandstone, in the Otsego Member of the Mount Marion Formation (Middle Devonian, Givetian) in eastern New York State, USA. Associated fossils consist mostly of rhynchonelliform brachiopods but also include bivalve molluscs, orthoconic nautiloids, linguliform brachiopods and gastropods. Many of the brachiopods, bivalve molluscs and conulariids have been buried in situ. Conulariids buried in situ are oriented with their aperture facing obliquely upward and with their long axis inclined at up to 87 degree to bedding. Most specimens are solitary, but some occur in V‐like pairs or in radial clusters consisting of three specimens, with the component specimens being about equally long or (less frequently) substantially different in length. The compacted apical end of Conularia buried in situ generally rests upon argillaceous sandstone. With one possible exception, none of the examined specimens terminates in a schott (apical wall), and internal schotts appear to be absent. The apical ends of specimens in V‐like pairs and radial clusters show no direct evidence of interconnection of their periderms. The apical, middle or apertural region of some inclined specimens abuts or is in close lateral proximity to a recumbent conulariid or to one or more spiriferid brachiopods, some of which have been buried in their original life orientation. The azimuthal bearings of Conularia and nautiloid long axes and the directions in which conulariids open are nonrandom, with conulariids being preferentially aligned between 350 and 50 degree and with their apertural end facing north‐east, and nautiloids being preferentially aligned between 30 and 70 degree. Otsego Member Conularia were erect or semi‐erect, epifaunal or partially infaunal animals, the apical end of which rested upon very fine bottom sediment. The origin of V‐like pairs and radial clusters remains enigmatic, but it is probable that production of schotts was not a regular feature of this animal’s life history. Finally, conulariids and associated fauna were occasionally smothered by distal storm deposits, under the influence of relatively weak bottom currents.     

Susceptibility of California gastropods to an introduced South African sabellid polychaete, Terebrasabella heterouncinata

Abstract. The outer surfaces of the shells of living marine gastropods are often colonized by other organisms. However, only one species, the sabellid worm Terebrasabella heterouncinata , is able to settle in the aperture of living gastropods. Native to South Africa, and introduced to California, this worm is a pest of abalone aquaculture and has been a threat to native gastropods in California. We investigated the intrinsic susceptibility of 15 marine gastropods from California to this apertural fouling organism. Intrinsic susceptibility was significantly different among gastropod species. Overall, caenogastropods tended to be more resistant than were the vetigastropods and patellogastropods. This suggests that variability in susceptibility could be due to characteristics associated with closely related gastropod hosts. However, this only partially explained the variation in susceptibility to individuals of T. heterouncinata . Intrinsic susceptibility was not associated with potential host species from similar habitats. We discuss host susceptibility to T. heterouncinata , including implications for potential control of this pest species, and for understanding factors enabling this polychaete to inhabit the apertural region, an area typically free of all other epibionts.     

SHELL MICROSTRUCTURE OF GASTROPODS FROM LAKE TANGANYIKA,AFRICA: ADAPTATION,CONVERGENT EVOLUTION,AND ESCALATION

Gastropod shells from Lake Tanganyika, with their heavy calcification, coarse noded ribbing, spines, apertural lip thickening and repair scars, resemble marine shells more closely than they resemble other lacustrine shells. This convergence between Tanganyikan and marine gastropod shells, however, is not just superficial. Scanning electron microscope (SEM) studies reveal that the Tanganyikan shells are primarily layers of crossed-lamellar crystal architecture (that is, needle-like aragonite crystals arranged into laths that are packed into sheets such that the aragonite needles of adjacent laths are never parallel). The number of crossed-lamellar layers can vary from one to four between different Tanganyikan gastropod species. In species with two or more crossed-lamellar layers, the orientation of the lamellae is offset by approximately 90° between the different layers. The number of crossed-lamellar layers in the shell wall is positively correlated with shell strength and with predation resistance. Three and four crossed-lamellar layers in the shell wall evolved several times independently within the endemic thiarid gastropod radiation in Lake Tanganyika. Repeated origins of three and four crossed-lamellar layers suggest that they may be specific adaptations by Tanganyikan gastropods to strengthen their shells as a defense against shell-crushing predators.